Many types of spinal irregularities can cause pain, limit range of motion, or injure the nervous system within the spinal column. These irregularities can result from, without limitation, trauma, tumor, disc degeneration, and disease. By way of example, scoliosis and kyphosis are irregularities that can result in a complex, three-dimensional problem with lateral curvature of the spine and segmental vertebral rotation. Advances in medicine and engineering have provided doctors with a plurality of devices and techniques for correcting these deformities or irregularities.
One technique for treating scoliosis is the Harrington rod. The Harrington rod operates on a distraction system attached by hooks to the spine that when distracted, straightens the curve in the spine. Until the 1970's, the Harrington rod was the preferred method of treatment for scoliosis. Follow-up studies of these rods documented the negative effects of using distraction rods alone for correction of spinal deformities, including “flat back syndrome” and the resulting degenerative changes in the lower lumbar spine. Other methods of management, including spinal fixations systems that include segmental wires, hooks, rods and/or bone fasteners (e.g., pedicle screws) have revolutionized the treatment of spinal deformities. The advent and availability of smaller and stronger bone fasteners has further advanced treatment, allowing for effective application of corrective forces to posterior systems into the high thoracic spine.
Until recently, spinal fixation systems comprising bone fasteners and rods were used almost exclusively to correct the lateral curvature of the spine, with little attention paid to segmental vertebral derotation, primarily because bone fastener instrumentation did not provide a means of rotating the individual vertebral bodies. These systems focused on reducing the rod into the bone fasteners and using rod grips and/or in situ benders to translate the rod to restore sagittal and coronal plane balance. While these patients appear to be corrected on an A-P or lateral x-ray, they frequently present with pronounced rib hump, or axial twisting of the spine, which can result in significant pulmonary compromise and further degenerative changes in the spine.
Techniques have also been developed to address segmental vertebral rotation caused by irregularities, such as scoliosis. For instance, derotation systems have been used for the treatment of segmental vertebral rotation. Current derotation systems typically include a tube or series of tubes that slot over the implant (e.g., pedicle screw) much like a counter torque. These tubes allow the surgeon to push on and rotate the vertebral bodies away from the instrumentation, which when applied to the apex of the deformity, will straighten the lateral curvature and can reduce some of the individual rotation between bodies. However, this maneuver frequently results in flat back syndrome and, more importantly, very high applied forces to individual screws, which can lead to bone fasteners breaching the medial or lateral walls of the pedicle. This leads to loss of fixation at these levels and can result in paralysis if the breached screws impact the spinal cord. To reduce the force applied to any one screw, derotation systems may link multiple screws together so that the force of correction is applied over multiple screws. However, the surgeon typically cannot connect more than two tubes, and the process to connect the tubes is time consuming, confusing, and often requires separate instruments to assemble and tighten the system together. These factors in an already long and complex deformity case have limited the impact of derotation systems for posterior spinal fusion procedures.
Thus, there is a need for improved derotation systems that can securely connect to implants with efficient coupling to multiple implants.